Webcam A/V Sync & Latency Meter

What is A/V sync and why it matters for streaming

A/V sync refers to the timing alignment between the audio and video tracks in a recording or live stream. When the two tracks are misaligned, viewers hear your voice before or after your lips move. Offsets around 80 to 150 ms are usually noticeable in live speech or streaming workflows and can make a broadcast feel unpolished regardless of how good the content is.¹

The offset is caused by different processing pipelines for video and audio inside the browser and the operating system. Your webcam's USB driver, the browser's video codec, and the JavaScript event loop all add latency to the video path. Your microphone's audio driver and the Web Audio API add a different amount to the audio path. The two rarely match.

How the clap detection algorithm works

The tool runs two independent detection loops simultaneously. The video loop draws each webcam frame to a hidden 320×240 canvas and samples the mean luminance of an 80×80 pixel region in the centre. When the frame-to-frame luminance delta exceeds a fixed threshold, the tool records a video timestamp using performance.now().² The brightness change that pushes the signal over the threshold is the flash of your hands completing the clap, so the timestamp lands close to the visible impact rather than the start of the hand motion.

The audio loop polls an AnalyserNode every 4 ms to read the peak amplitude deviation from silence. During the first 800 ms after you click Start Test, the tool calibrates a noise floor from ambient room sound. A transient is detected when the peak exceeds the noise floor by a factor of 3.5, and an audio timestamp is recorded using the same performance.now() clock so both measurements share a single time base.³

How the timestamps are paired and averaged

The two timestamps are paired only when they fall within a 500 ms correlation window, which is wide enough to absorb the real-world offset between a camera frame and its matching audio transient while still rejecting unrelated background events. The delta is audioTs − videoTs: a positive value means audio arrived after video, and a negative value means audio arrived first. After five valid pairs, the tool averages the deltas and reports the result, which smooths out any single frame that arrived late or any clap that landed closer to one detector than the other.

Reading your result and entering it in OBS

The result is shown as a signed millisecond value with a direction label: Audio Delay means audio arrives after video, so the sound trails the picture; Audio Leads means audio arrives before video, so the sound jumps ahead of the picture; and In Sync means the offset is within ±5 ms and no correction is needed because the two tracks are already close enough to appear simultaneous to viewers.

Applying the value in OBS Studio

In OBS Studio, open Edit → Advanced Audio Properties and find your microphone source in the list. Enter the OBS Audio Offset value from this tool in the Sync Offset (ms) column: a positive offset pushes the audio track forward in time so it lines up with later-arriving video frames, while a negative offset delays the audio to wait for the picture. Click Close, then verify the fix by clapping once more with your stream preview open; the visual slap and the sound should now reach the viewer at the same instant.

A concrete example makes the direction easier to remember. If your five-clap test reports Audio Delay: +80 ms, your microphone's signal is arriving 80 milliseconds behind the camera's, which sits inside the 80 to 150 ms range viewers start to notice in live speech. Enter 80 in OBS Studio's Sync Offset field for that microphone source, then clap once more with the stream preview open. A corrected setup lands within the small window this tool treats as in sync, so the visual slap and the sound should now reach the viewer together.

Browser support

The tool requires navigator.mediaDevices.getUserMedia with both video and audio constraints, plus the Web Audio API. All major browsers support this: Chrome 53+, Firefox 36+, Edge 12+, and Safari 11+. Because getUserMedia is a privacy-sensitive API, the page must be loaded over HTTPS, from localhost, or from a file:// URL before the browser will even expose the media devices, and the browser will then prompt for camera and microphone permissions on first use.⁴

Where the media data goes during a test

No audio or video data is sent to a server at any point during the test. The camera and microphone streams exist only inside your browser tab's memory, where the video loop reads each frame from a canvas and the audio loop reads amplitude from a local AnalyserNode, so the raw media never touches the network stack or a remote endpoint. The stream is released and the device handles are freed when you close the tab or navigate away from the page, which means you can run the measurement and walk away without leaving any recorded file behind.

What changes your measured A/V offset

Your measured offset depends on the exact hardware and software stack in use at the time of measurement. Changing webcam, microphone, driver version, browser, or operating system can shift the reading by tens of milliseconds between sessions.⁵ A USB audio interface contributes additional buffer and driver latency, and larger buffers increase the delay.⁴ Bluetooth audio devices introduce the most variability: standard Bluetooth codecs can add substantial delay, while aptX Low Latency reduces this to about 40 ms and AAC still adds more delay than a wired USB microphone.⁶⁷

Remeasure your offset whenever your hardware setup changes or after a major browser or operating system update. A new browser version may change the internal processing pipeline for camera capture or audio routing, shifting the reading from its previous value. The five-clap test completes in under a minute and ensures the value you enter in OBS or your conferencing app remains accurate for your current exact configuration, rather than carrying a stale reading from a different setup. CapyToolkit runs the entire analysis in browser memory and never stores or uploads any audio or video data.